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Water reservoirs play a decisive role in the distribution of drinking water (consumption, irrigation, etc.) and in the production of electrical energy, and at the same time have a huge impact on the operation of the rivers where they are located. Traditionally, reservoirs are considered net carbon sinks that accumulate more carbon than they emit into the atmosphere. Now, a new study upends the traditional assumption of the ecological role of reservoirs in the global carbon cycle, revealing that they emit twice as much carbon as they capture. Professor Biel Obrador, from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona, and other experts from the Helmholtz Research Center participate in the study, published in the journal Nature Geosciences Environmental in Magdeburg (UFZ, Germany) and the Catalan Water Research Institute (ICRA) in Girona. The work has been carried out within the framework of the C-HydroChange project of the National Plan of the Ministry of Science, Innovation and Universities.
River networks transport large amounts of organic matter from terrestrial ecosystems to the oceans. Part of this organic matter is metabolized by aquatic communities and is emitted into the atmosphere as carbon dioxide (CO 2 ) or methane (CH 4 ), two gases with an important greenhouse effect. When a river flows into a reservoir, this organic material gradually accumulates in the sediments. “There is a large amount of organic carbon permanently buried in the sediments of the reservoirs. Until now, it was considered that reservoirs buried more carbon than they emitted”, explains expert Phillip Keller, member of the Helmholtz Center for Environmental Research, in Germany (UFZ), and first author of the study.
A new scenario for calculating the global balance of the carbon cycle
To calculate the carbon balance in water bodies, not only flooded areas must be taken into account, but also those areas that dry out when the water level drops. When the sediment comes into contact with the atmosphere, mineralization processes are stimulated and a lot of carbon dioxide is emitted, as previous studies in reservoirs, lakes, rivers and ponds carried out by the UB-IRBio teams have found. ICRA and UFZ. “When they dry out, sediments emit much more carbon than areas covered by water”, explains Biel Obrador, who is a member of the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology and the Forestream Research Group of the UB
“When the reservoir’s water level drops, large areas dry up. Until now, these areas had not been considered in the calculation of the carbon balance of the reservoirs. Our study aims to close this gap in knowledge”, says Obrador.
As part of the work, the researchers used a database of satellite images with information available from nearly 6,800 reservoirs around the world from 1985 to 2015. “For each reservoir there is monthly data for a period of 30 years. So, we could quantify exactly when, where and for how long the reservoirs were not completely full and determine the extent of the dried surface”, explains expert Matthias Koschorreck, also a member of the Helmholtz Center for Environmental Research (UFZ) .
On average, 15% of the total surface of the reservoirs was not covered by water. With these figures, the scientists recalculated the global carbon emission from the reservoirs. “Our calculations show that carbon emissions have been clearly underestimated in the past. As a global average, reservoirs emit twice the amount of carbon they bury. Therefore, it is necessary to rethink its image as a clean carbon sink», underlines Biel Obrador.
The study’s findings also reveal that water level fluctuations depend on both the use of the reservoir and its geographic location. “Irrigation reservoirs fluctuate more than hydroelectric reservoirs. And in places with less seasonal fluctuations in precipitation -closer to the poles and the equator- the water level is more stable compared to intermediate latitudes”, emphasizes Rafael Marcé, member of the Catalan Research Institute Water (ICRA) and the University of Girona.
Experts point to the fact that the flow of carbon in dry areas would affect the global carbon balance of continental waters. “We hope that our study will increase awareness that dry areas must be considered in the global carbon balance of continental waters”, emphasize the experts. In addition, the results can drive new climate-compatible reservoir management strategies. “If it is necessary to reduce the water level of a reservoir to carry out maintenance work, it would be advisable to consider what is the best period to do so. If the work is carried out in the cold season, the mineralization processes are slower and less carbon is emitted from the dry areas”, point out the authors.
The study of greenhouse gas emissions from reservoirs – in particular, completely dried reservoirs after being dismantled – focuses on one of the research lines of the UB-IRBio team. “All over the world, many reservoirs are approaching the end of their useful life as infrastructure. What is the carbon balance of a reservoir when the dam is removed and the reservoir is emptied? We hope to give clues about this in future works”, concludes Biel Obrador.
